Real-time arc control in electrosurgical generators

ABSTRACT

An electrosurgical generator is disclosed. The generator includes a radio frequency output stage configured to generate a radio frequency waveform and a sensor circuit configured to measure a property of the radio frequency waveform during a predetermined sampling period to determine whether an arc event has occurred. The generator also includes a controller configured to determine a total charge and/or total energy deposited by the radio frequency waveform during the predetermined sampling period associated with the arc event. The controller is further configured to adjust the output of the electrosurgical generator based on at least one parameter to limit arcing.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patent application Ser. No. 11/859,039 filed on Sep. 21, 2007, the entire contents of which are incorporated by reference herein.

BACKGROUND

1. Technical Field

The present disclosure relates to electrosurgical apparatuses, systems and methods. More particularly, the present disclosure is directed to electrosurgical generators configured to control energy output on a per arc basis. The generator detects aberrations within continuous or pulsed waveforms indicative of sudden changes in the current. In particular the generator monitors for deviations from the sinusoidal or pulsed sinusoidal current waveforms as compared to the voltage waveform.

2. Background of Related Art

Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, heat, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, an active electrode delivers radio frequency energy from the electrosurgical generator to the tissue and a return electrode carries the current back to the generator. In monopolar electrosurgery, the active electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator.

Ablation is most commonly a monopolar procedure that is particularly useful in the field of cancer treatment, where one or more RF ablation needle electrodes (usually having elongated cylindrical geometry) are inserted into a living body and placed in the tumor region of an affected organ. A typical form of such needle electrodes incorporates an insulated sheath from which an exposed (uninsulated) tip extends. When an RF energy is provided between the return electrode and the inserted ablation electrode, RF current flows from the needle electrode through the body. Typically, the current density is very high near the tip of the needle electrode, which tends to heat and destroy surrounding issue.

In bipolar electrosurgery, one of the electrodes of the hand-held instrument functions as the active electrode and the other as the return electrode. The return electrode is placed in close proximity to the active electrode such that an electrical circuit is formed between the two electrodes (e.g., electrosurgical forceps). In this manner, the applied electrical current is limited to the body tissue positioned immediately adjacent the electrodes. When the electrodes are sufficiently separated from one another, the electrical circuit is open and thus inadvertent contact with body tissue with either of the separated electrodes does not cause current to flow.

During electrosurgical procedures, the magnitude and temporal characteristics of the voltage and current as supplied by the electrosurgical generator determine energy density pathways and tissue temperatures. This may be accomplished by keeping the total delivered power constant while varying other energy properties, such as voltage, current, etc. These configurations are not configured for detecting and controlling arcing conditions between the active electrode and tissue. It is particularly desirable to prevent the occurrence of uncontrolled electrical arcs and concomitant energy deposition on a half cycle or shorter time scale, in order to avoid inadvertent tissue damage and to achieve optimum conditions. Thus, there is a continual need for electrosurgical generators which are configured to sense tissue and energy properties to determine arcing conditions and control energy output based on these determinations.

SUMMARY

According to one aspect of the present disclosure, an electrosurgical generator is disclosed. The generator includes a radio frequency output stage configured to generate a radio frequency waveform and a sensor circuit configured to measure a property of the radio frequency waveform during a predetermined sampling period. The generator also includes a controller configured to determine a total charge transported and/or total energy deposited by the radio frequency waveform during the predetermined sampling period to determine an arc event. In particular the arc duration of special interest as determined by the deviation of the current and voltage waveforms, as well as the integral of this difference to determine the total change in the arc. Arcing can be characterized by an uncontrolled disparity between the voltage and current waveforms with the current waveform being much larger for a short period compared to the characteristic waveforms. The controller is further configured to adjust the output of the electrosurgical generator based on at least one parameter associated with the arc event.

A method for operating an electrosurgical generator is also contemplated by the present disclosure. The method includes the steps of selecting duration of sampling period, measuring voltage and current of a radio frequency waveform having a current waveform and a voltage waveform across a series resistor of the electrosurgical generator during the sampling period and temporally integrating voltage across the series resistor to determine total charge deposited into tissue by the radio frequency waveform during the sampling period and in particular during the arc duration. The method also includes the steps of temporally integrating power of the at least one radio frequency waveform during the sampling period to determine total energy deposited into tissue by the radio frequency waveform during the sampling period, by isolating the interval during which the current waveform is different from the voltage waveform to determine energy per arc event as a function of the total charge and total energy delivered and determining the amount of energy to be deposited into tissue per arc event.

According to a further embodiment of the present disclosure, an electrosurgical generator is disclosed. The electrosurgical generator includes a radio frequency output stage configured to generate a radio frequency waveform having a current waveform and a voltage waveform, a sensor circuit configured to measure voltage and current of a radio frequency waveform during a predetermined sampling period, and a controller configured to compare the voltage waveform and the current waveform and to provide a signal representative of the comparison. The controller includes a total charge calculator module configured to temporally integrate current of the radio frequency waveform and in particular the current during the interval when the current waveform departs from the voltage waveform to determine the total charge deposited by the radio frequency waveform during the predetermined sampling period as well as during the arc events. The controller also includes a total energy calculator module configured to integrate total power deposited by the radio frequency waveform to determine the total energy deposited by the radio frequency waveform during the predetermined sampling period for the radio frequency waveform and for arc events. The controller is further configured to determine an arc event as a function of the total charge and total energy and to adjust the output of the electrosurgical generator if the arc event is determined to avoid undesired tissue damage by setting a threshold energy above which the arc event is extinguished.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein with reference to the drawings wherein:

FIGS. 1A-1B are schematic block diagrams of an electrosurgical system according to the present disclosure;

FIG. 2 is a schematic block diagram of a generator according to one embodiment of the present disclosure; and

FIG. 3 is a flow chart illustrating a method for determining energy of a radio frequency waveform and arc events thereof according to the present disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.

The generator according to the present disclosure can perform monopolar and bipolar electrosurgical procedures, including vessel sealing procedures. The generator may include a plurality of outputs for interfacing with various electrosurgical instruments (e.g., a monopolar active electrode, return electrode, bipolar electrosurgical forceps, footswitch, etc.). Further, the generator includes electronic circuitry configured for generating radio frequency power specifically suited for various electrosurgical modes (e.g., cutting, blending, division, etc.) and procedures (e.g., monopolar, bipolar, vessel sealing).

FIG. 1A is a schematic illustration of a monopolar electrosurgical system 1 according to one embodiment of the present disclosure. The system 1 includes an electrosurgical instrument 2 having one or more electrodes for treating tissue of a patient P. The instrument 2 is a monopolar type instrument including one or more active electrodes (e.g., electrosurgical cutting probe, ablation electrode(s), etc.). Electrosurgical RF energy is supplied to the instrument 2 by a generator 20 via a supply line 4, which is connected to an active terminal 30 (FIG. 2) of the generator 20, allowing the instrument 2 to coagulate, seal, ablate and/or otherwise treat tissue. The energy is returned to the generator 20 through a return electrode 6 via a return line 8 at a return terminal 32 (FIG. 2) of the generator 20. The active terminal 30 and the return terminal 32 are connectors configured to interface with plugs (not explicitly shown) of the instrument 2 and the return electrode 6, which are disposed at the ends of the supply line 4 and the return line 8, respectively.

The system 1 may include a plurality of return electrodes 6 that are arranged to minimize the chances of tissue damage by maximizing the overall contact area with the patient P. In addition, the generator 20 and the return electrode 6 may be configured for monitoring so-called “tissue-to-patient” contact to insure that sufficient contact exists therebetween to further minimize chances of tissue damage. In one embodiment, the active electrode 6 may be used to operate in a liquid environment, wherein the tissue is submerged in an electrolyte solution.

FIG. 1B is a schematic illustration of a bipolar electrosurgical system 3 according to the present disclosure. The system 3 includes a bipolar electrosurgical forceps 10 having one or more electrodes for treating tissue of a patient P. The electrosurgical forceps 10 include opposing jaw members having an active electrode 14 and a return electrode 16, respectively, disposed therein. The active electrode 14 and the return electrode 16 are connected to the generator 20 through cable 18, which includes the supply and return lines 4, 8 coupled to the active and return terminals 30, 32, respectively (FIG. 2). The electrosurgical forceps 10 are coupled to the generator 20 at a connector 21 having connections to the active and return terminals 30 and 32 (e.g., pins) via a plug disposed at the end of the cable 18, wherein the plug includes contacts from the supply and return lines 4, 8.

The generator 20 includes suitable input controls (e.g., buttons, activators, switches, touch screen, etc.) for controlling the generator 20. In addition, the generator 20 may include one or more display screens for providing the user with variety of output information (e.g., intensity settings, treatment complete indicators, etc.). The controls allow the user to adjust power of the RF energy, waveform, as well as the level of maximum arc energy allowed which varies depending on desired tissue effects and other parameters to achieve the desired waveform suitable for a particular task (e.g., coagulating, tissue sealing, intensity setting, etc.). The instrument 2 may also include a plurality of input controls that may be redundant with certain input controls of the generator 20. Placing the input controls at the instrument 2 allows for easier and faster modification of RF energy parameters during the surgical procedure without requiring interaction with the generator 20.

FIG. 2 shows a schematic block diagram of the generator 20 having a controller 24, a high voltage DC power supply 27 (“HVPS”) and an RF output stage 28. The HVPS 27 is connected to a conventional AC source (e.g., electrical wall outlet) and provides high voltage DC power to an RF output stage 28, which then converts high voltage DC power into RF energy and delivers the RF energy to the active terminal 30. The energy is returned thereto via the return terminal 32.

In particular, the RF output stage 28 generates either continuous or pulsed sinusoidal waveforms of high RF energy. The RF output stage 28 is configured to generate a plurality of waveforms having various duty cycles, peak voltages, crest factors, and other suitable parameters. Certain types of waveforms are suitable for specific electrosurgical modes. For instance, the RF output stage 28 generates a 100% duty cycle sinusoidal waveform in cut mode, which is best suited for ablating, fusing and dissecting tissue and a 1-25% duty cycle waveform in coagulation mode, which is best used for cauterizing tissue to stop bleeding.

The radio frequency waveforms include a current and a voltage waveform. The present disclosure provides for a system and method which monitors and compares the voltage and current waveform to detect discrepancies between the waveform on a time scale substantially equal to one-half radio frequency cycle of the waveform.

The generator 20 may include a plurality of connectors to accommodate various types of electrosurgical instruments (e.g., instrument 2, electrosurgical forceps 10, etc.). Further, the generator 20 may operate in monopolar or bipolar modes by including a switching mechanism (e.g., relays) to switch the supply of RF energy between the connectors, such that, for instance, when the instrument 2 is connected to the generator 20, only the monopolar plug receives RF energy.

The controller 24 includes a microprocessor 25 operably connected to a memory 26, which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The microprocessor 25 includes an output port that is operably connected to the HVPS 27 and/or RF output stage 28 allowing the microprocessor 25 to control the output of the generator 20 according to either open and/or closed control loop schemes. Those skilled in the art will appreciate that the microprocessor 25 may be substituted by any logic processor or analog circuitry (e.g., control circuit) adapted to perform the calculations discussed herein.

The generator 20 may implement a closed and/or open loop control schemes which include a sensor circuit 22 having a plurality of sensors measuring a variety of tissue and energy properties (e.g., tissue impedance, tissue temperature, output current and/or voltage, etc.), and providing feedback to the controller 24. A current sensor can be disposed at either the active or return current path or both and voltage can be sensed at the active electrode(s). The controller 24 compares voltage and current waveforms to identify arc events, the duration thereof and total energy of the arc event. The controller 24 then transmits appropriate signals to the HVPS 27 and/or RF output stage 28, which then adjust DC and/or RF power supply, respectively by using a maximum allowable arc energy which varies according to the selected mode. The controller 24 also receives input signals from the input controls of the generator 20 or the instrument 2. The controller 24 utilizes the input signals to adjust power output by the generator 20 and/or performs other control functions thereon.

The sensor circuit 22 measures the electrical current (I) and voltage (V) supplied by the RF output stage 28 in real time to characterize the electrosurgical process during both the matching sinusoidal and non-sinusoidal durations for a predetermined sampling period, the former being of short duration (e.g., half a cycle) and the latter being of long duration (e.g., 15 cycles). This allows for the measured electrical properties to be used as dynamic input control variables to achieve feedback control. The current and voltage values may also be used to derive other electrical parameters, such as power (P=V*I) and impedance (Z=V/I). The sensor circuit 22 also measures properties of the current and voltage waveforms and determines the shape thereof.

More specifically, the controller 20 includes a total charge calculator module 40 and a total energy calculator 42. The total charge calculator module 40 is configured to determine the total charge, “q,” delivered to the tissue in Coulombs both on average and during an arc event. This is accomplished by integrating the measured current over the two predetermined sampling periods, the sinusoidal and non-sinusoidal durations. The total energy calculator module 42 determines the total energy, “E,” delivered into the tissue during treatment. The total energy calculator module 42 determines the power delivered by generator 20 and then integrates the power over the sampling period. The sampling period can range from about a small fraction of the half-cycle of the electrosurgical waveform during arcs to a plurality of full cycles.

The controller 20 is also configured to determine deviations between one or more properties of the voltage waveform and the current waveform by comparing the waveforms. As stated above, a discrepancy between the voltage and current waveforms is indicative of an arc event, this allows the controller 20 to determine when an arc event has occurred and then utilize the calculators 40 and 42 to determine the total charge, total energy and duration of the arc event. In other words, once an arc even has been detected based on the comparison of the waveforms, the controller 20 thereafter generates a comparison signal and performs total charge and energy calculations for the period of time corresponding to the deviation (e.g., the arc event).

The generator 20 also includes a circuit 50, having a series inductance, a series resistance and a shunt capacitance which are shown schematically as a series inductor 46, a series resistor 48 and a shunt capacitor 49. The inductor 46 and resistor 48 are disposed on the active terminal 30, with the capacitor 49 being disposed between the active and return terminals 30 and 32. Inductance and capacitance of these components is determined by characteristics of the load elements (e.g., the active electrode 2, and the tissue). The resistance is selected for a so-called quality factor, “Q,” corresponding to the excitation of the circuit 50. The resistance, inductance and capacitance are also selected to accommodate system and parasitic electrical properties associated with the generator 20 (e.g., inductance and capacitance of the cable 4).

The total charge calculator module 40 determines the total charge, “q,” by temporal integration of the voltage across the resistor 48. The charge arises from the stored energy in reactive components described above and available for the arc duration when this energy is suddenly discharged. The sensor circuit 22 measures the voltage at the resistor 48 for the desired sampling period and compares the voltage to the current waveform. The sensor circuit 22 then transmits the voltage, current and time values of the sampling period to the total charge calculator module 40, which thereafter integrates the values to obtain “q.” The total energy calculator module 42 determines the total energy, “E,” by temporal integration of the product of “q” and the voltage across the capacitor 49 during an arc event. In particular, the controller 24 is configured to utilize the results of the determinations by the total charge calculator module 40 and the total energy calculator module 42 to adjust the operation of the generator 20 during the arc-free and arc events. For every detected arc event, the controller 24 is configured to set a maximum energy to be delivered by the RF output stage 28 per instantaneous arc based on formula (1):

E=CV ²/2+LI ²/2  (1)

In formula (1), E is the desired energy, C is the capacitance of the capacitor 49, L is the inductance of the inductor 46, and V is voltage measured across thereof.

In another embodiment, arcing maybe controlled by placing limits on operational parameters in conjunction or in lieu of active feedback control. The limits are based on media properties (e.g., liquid medium in which the tissue is submerged) and mobile charge (e.g., electrolyte) concentrations. Mobile charges in the media can be either positive or negative. Further, the charges move through the media at a different rate defined in units of velocity per electric field. During ideal operation the balance between the positive and negative charges can be expressed by the formula (2):

(mobility of negative charges)*(positive voltage magnitude)*(duration of positive voltage)=(mobility of positive species)*(negative voltage magnitude)*(duration of negative voltage)  (2)

The equilibrium between the charges occurs when positive and negative charges travel the same distance, such that there is no accumulation of the faster-moving charges at the surface of the active electrode. Accumulation of faster species at the instance when the voltage switches polarity results in deleteriously high charge density at the tip of the active electrode. The spatial gradient of the charge density gives rise to a potential that is high enough to cause a breakdown which results in arcing. Thus, arcing can be minimized by putting limits on operational parameters of the generator 20 to produce desired waveforms which match or equalize charge transport of each polarity of species. Namely, the voltage, current, and other output parameters of the generator 20 are adjusted as a function of the formula (2) based on prior empirical measurements or experience to decrease the amount of faster charges at the tip of the active electrode when the voltage switches polarity.

FIG. 3 illustrates a method for adjusting energy per arc in response to total charge and total energy values for arc events and arc-free intervals. In step 100, the controller 24 selects duration for the sampling period, during which the sense circuit 20 is going to measure voltage, current, and other tissue and/or waveform properties. The length of the sampling period can be for the duration of a sub-cycle (e.g., half-cycle) or a plurality of cycles of the waveform, depending on the desired breadth of the value.

In step 102, the sensor circuit 22 measures the voltage across the resistor 48 for the duration of the sampling period. In step 104, the total charge calculator module 40 determines the total charge deposited into the tissue by temporally integrating the current arc waveform at the resistor 48 with respect to the length of the sampling period and deviating from the voltage waveform's sinusoidal character. In addition to the total charge determination, the controller 24 also determines the total energy deposited during the sampling period of the short duration arc. This is accomplished in step 106 by measuring the voltage across the capacitor 49 during the sampling period and comparing the voltage waveform with the current waveform deviations to determine the duration of any arc event and the energy delivered by the arc. In particular, the controller 24 compares the shape of the voltage waveform and the current waveform to determine a deviation between the shape of the voltage waveform and the current waveform indicative of an arc event. In step 108, the total energy calculator module 42 multiplies the shunt capacitance voltage and the total charge calculated in step 104 to determine total energy.

In another embodiment, as illustrated in step 107, the total energy calculator module 42 determines the total energy amount in both the arc free duration and during the arc event by determining the power supplied to the tissue and thereafter temporally integrating the power value deposited to the tissue during the sampling period. In step 110, the controller 24 determines a desired amount of energy to be deposited into tissue per arc event by using the formula (1). In step 112, the controller 24 signals the RF output stage 28 to output the desired energy level on a per arc basis. This allows the generator 20 to tailor the output to avoid arcing beyond a predetermined set point depending on the selected electrosurgical mode. Thus, as an arcing event is detected, the output is adjusted accordingly to avoid tissue damage.

In embodiments, steps 102 and 104 can run concurrently with steps 106 and 108, such that total charge and total energy calculations are performed simultaneously and the output of these calculations are provided in parallel to the controller 24 in step 110.

While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto. 

1. An electrosurgical generator comprising: a radio frequency output stage configured to generate at least one radio frequency waveform having a voltage waveform and a current waveform; a sensor circuit configured to determine at least one property of each of the voltage waveform and the current waveform during a predetermined sampling period; and a controller configured to compare the at least one property of each of the voltage waveform and the current waveform on a time scale substantially equal to one-half radio frequency cycle of the at least one radio frequency waveform to determine at least one deviation between the at least one property of each of the voltage waveform and the current waveform indicative of an arc event, the controller further configured to determine at least one of total charge, total energy and duration of the arc event based on the at least one deviation.
 2. The electrosurgical generator according to claim 1, wherein the controller is further configured to adjust the output of the electrosurgical generator based on the at least one deviation.
 3. The electrosurgical generator according to claim 2, wherein the controller includes a total charge calculator module configured to temporally integrate the voltage waveform to determine the total charge deposited by the at least one radio frequency waveform during the predetermined sampling period.
 4. The electrosurgical generator according to claim 2, wherein the controller includes a total energy calculator module configured to temporally integrate total power deposited by the at least one radio frequency waveform to determine the total energy deposited by the at least one radio frequency waveform during the predetermined sampling period.
 5. The electrosurgical generator according to claim 1, further comprising: a shunt capacitor disposed between active and return terminals of the electrosurgical generator.
 6. The electrosurgical generator according to claim 5, wherein the controller includes a total energy calculator module configured to integrate product of total charge deposited by the at least one radio frequency waveform and voltage across the shunt capacitor to determine the total energy deposited by the at least one radio frequency waveform during the predetermined sampling period.
 7. The electrosurgical generator according to claim 5, wherein the controller is configured to determine a desired amount of energy to be deposited into tissue per arc event based on a formula E=CV²/2+LI²/2, wherein E is the desired energy, C is capacitance of the shunt capacitor, L is inductance of a series inductor, I is current and V is voltage measured across the shunt capacitor.
 8. A method for operating an electrosurgical generator, comprising the steps of: generating at least one radio frequency waveform having a voltage waveform and a current waveform; determining at least one property of each of the voltage waveform and the current waveform during a predetermined sampling period; measuring amplitude of the voltage waveform across a series resistor of the electrosurgical generator during the sampling period; comparing the at least one property of each of the voltage waveform and the current waveform on a time scale substantially equal to one-half radio frequency cycle of the at least one radio frequency waveform to determine at least one deviation between the at least one property of each of the voltage waveform and the current waveform indicative of an arc event; and determining at least one of total charge, total energy and duration of the arc event based on the at least one deviation.
 9. The method according to claim 12, wherein the step of calculating the desired amount of energy is based on a formula E=CV²/2+LI²/2, wherein E is the desired energy, C is capacitance of the shunt capacitor, L is inductance of a series inductor, I is current and V is voltage measured across the shunt capacitor.
 10. The method according to claim 8, further comprising the steps of: temporally integrating voltage across the series resistor to determine total charge deposited into tissue by the at least one radio frequency waveform during the sampling period; and temporally integrating power of the at least one radio frequency waveform during the sampling period to determine total energy deposited into tissue by the at least one radio frequency waveform during the sampling period.
 11. The method according to claim 9, further comprising the steps of: measuring voltage across a shunt capacitor disposed between active and return terminals of the electrosurgical generator during the sampling period; and temporally integrating a product of the total charge and the voltage across the shunt capacitor to determine total energy deposited into tissue by the at least one radio frequency waveform during the sampling period.
 12. The method according to claim 11, further comprising the steps of: calculating desired amount of energy to be deposited into tissue per arc event to limit the arc event; and adjusting output of the electrosurgical generator based on the desired maximum amount of energy to be deposited into tissue per arc event.
 13. An electrosurgical generator comprising: a radio frequency output stage configured to generate at least one radio frequency waveform having a current waveform and a voltage waveform; a sensor circuit configured to determine at least one property of each of the voltage waveform and the current waveform during a predetermined sampling period; and a controller configured to compare the voltage waveform and the current waveform on a time scale substantially equal to one-half radio frequency cycle of the at least one radio frequency waveform and to provide a comparison signal representative of the comparison, the controller also includes a total charge calculator module configured to temporally integrate voltage of the at least one radio frequency waveform to determine the total charge deposited by the at least one radio frequency waveform during the predetermined sampling period, the controller further including a total energy calculator module configured to integrate total power deposited by the at least one radio frequency waveform to determine the total energy deposited by the at least one radio frequency waveform during the predetermined sampling period, wherein the controller is configured to determine the total charge and total energy in response to the comparison signal.
 14. The electrosurgical generator according to claim 13, further comprising: a shunt capacitor disposed between active and return terminals of the electrosurgical generator.
 15. The electrosurgical generator according to claim 14, wherein the controller includes a total energy calculator module configured to integrate product of total charge deposited by the at least one radio frequency waveform and voltage across the shunt capacitor to determine the total energy deposited by the at least one radio frequency waveform during the predetermined sampling period.
 16. The electrosurgical generator according to claim 13, wherein the controller is configured to determine a desired amount of energy to be deposited into tissue per arc event based on a formula E=CV²/2, wherein E is the desired energy, C is capacitance of the shunt capacitor and V is voltage measured across thereof.
 17. The electrosurgical generator according to claim 5, further comprising: a resistor and an inductor coupled in series with the active terminal. 